How much battery sag is too much for a 3 year old battery?

Ghost1951

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Jun 2, 2024
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My bafang BBS01 has spent more time in the garage than on the road over the last three years. It has done about 2000 miles and has been kept charged when not in use.

Now that I have tinkered with the firmware settings so I can get easy access to higher power levels in my hilly location, I am noticing a fair bit of voltage sag when pulling more current. I know sag is part of the game, but how much is too much?

So to give a bit of context:

After 24 miles of riding around the hilly locale, the resting voltage of the 10s 6p Sanyo battery was 35 volts.

Using 250 watts up a slope the battery sagged to 34 volts.
Drawing 500 watts it sagged to 32 volts.

Given a short break either by me doing the work or switching it off for a minute the voltage returned to 35 volts off load, 34 volts on a 250 watt load and 32 volts when pulling 500 watts.

Time for a new battery, or just ride it?

Obviously, it isn't new, so it is expected that some performance penalty arises.

5810958108
 

matthewslack

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Nov 26, 2021
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35V is less than half charge. I would just ride it, and when voltage gets down to that level, keep power demand down to limit sag.

Maybe plan for your next battery, purchase before the old one fails, then use the old one when you can, keeping the new one for the more demanding outings.
 
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Ghost1951

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Jun 2, 2024
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Thanks Matthew.

Yes, It had done a fair mileage - over 20, before I noticed the sag, and as soon as I backed off on power demand it perked up no end. I am charging the battery now and will re-test when it is up at 40 volts to see how much sag that level of power demand causes when the battery is freshened up.

Cheers.
 

saneagle

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Oct 10, 2010
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My bafang BBS01 has spent more time in the garage than on the road over the last three years. It has done about 2000 miles and has been kept charged when not in use.

Now that I have tinkered with the firmware settings so I can get easy access to higher power levels in my hilly location, I am noticing a fair bit of voltage sag when pulling more current. I know sag is part of the game, but how much is too much?

So to give a bit of context:

After 24 miles of riding around the hilly locale, the resting voltage of the 10s 6p Sanyo battery was 35 volts.

Using 250 watts up a slope the battery sagged to 34 volts.
Drawing 500 watts it sagged to 32 volts.

Given a short break either by me doing the work or switching it off for a minute the voltage returned to 35 volts off load, 34 volts on a 250 watt load and 32 volts when pulling 500 watts.

Time for a new battery, or just ride it?

Obviously, it isn't new, so it is expected that some performance penalty arises.

View attachment 58109View attachment 58108
That sounds pretty good to me. I think many people would be happy to get only 3v sag at 500w.

That crank motor's going to have to go, though. I'd be more worried about that.
 

Ghost1951

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Jun 2, 2024
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That sounds pretty good to me. I think many people would be happy to get only 3v sag at 500w.

That crank motor's going to have to go, though. I'd be more worried about that.
Thanks for the info and the warning..... But... Why?

I'm loving that motor now I have it legal. Cuts off at 15mph, plenty of power to go up hill, and silent as befits my name - Ghostly quiet. Maybe you should have got a Bafang instead of the other one. I can also Ghost pedal and it flies up hill. Spookey....


As it happens, on the way back just now, a chunky police pickup passed me on the little road to the next village where I had been fixing the Gf's toilet. Damned water fill valve was letting water through. He didn't even see me. My cloak of invisibility I expect.

What is it about those siamp fill valves. Always going wrong. You take them apart and there is nothing amiss. Put them back together and they work again. Bloody attention seeking things...


Edit - oh and I was able to disable the throttle in the firmware, by setting it to current control and then setting current at zero. Doesn't work now. I'm fine with that. I don't need it.
 

Peter.Bridge

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Apr 19, 2023
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What's the capacity of your battery ? On very hilly rides, I get that sort of sag (I have a 48v 20Ah battery). I do try and plan my rides to do the steepest hills on the way out ! I don't think your battery is on its way out, but it might be too small now you are using the motor to it's capacity, depending on the length and profile of your rides.
 
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guerney

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Sep 7, 2021
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My 19.2Ah battery sags a lot sometimes, and I enjoy watching it sag... but unless the motor cuts out it really isn't a problem - that has only happened a couple of times, uphill when the batterry was nearly pooped and the motor was running on lithium fumes. I hate to say the second time it happened, I was forced to lower assistance from maximum/like a stabbed rat up drainpipe level.

However, increasing the controller limit to 18A and using it to climb hills, results in a warmer battery. My battery is over three years old - I should really stick to 15A and reserve the extra 3A for emergencies like this one, when a little more hill climbing speed would have made the substance maddened Gremlins chasing on foot uphill part of the situation a little less stressful.




Battery sag is visible on the display, but it wasn't much. Sags more now with the controller at 18A... but all hill climbs are faster, and for most small hills, the motor cuts power when I hit 25kph, starts again at about 24 or 23 kph, back up to 25kph etc - hill climb speed is limited on most small hills by the pedal assist speed limit. Easy breezy zero effort pedelecing, provided you can keep your legs moving a bit, which might suit some ill/elderly/unfit/lazy people... and people like me who prefer to arrive at work sweat free and full of beans. Reminds me, must make more natto.
 
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Ghost1951

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Jun 2, 2024
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What's the capacity of your battery ? On very hilly rides, I get that sort of sag (I have a 48v 20Ah battery). I do try and plan my rides to do the steepest hills on the way out ! I don't think your battery is on its way out, but it might be too small now you are using the motor to it's capacity, depending on the length and profile of your rides.
I think it is about 13Ahr. 36 volts. Going by the numbers I see on the display, I see about 550 watts when the voltage is around 39 volts. This comes out at about 14 amps draw on the battery. The controller is currently set to a maximum of 16 amps. I have experimented a bit with the settings and the wattage figure ought to represent the motor working to its maximum. It was uphill and I had the cadence right for it to peak its power. The power is plenty for any use I can see around these parts, but I think the three year old battery clearly can't put out the 16 amps asked of it. That would account for the voltage sag I think.

I would say that the battery probably has a lot of use in it still, but it is clearly not at its best at this point. It doesn't need to be. I can live with it fine for knocking about. If I wanted to do very long rides in tough country, I'd need to get a new battery, and probably a bigger one. I could fit a rack and a box for carrying a spare battery to swap out if need be. I doubt that I will though Peter.

Cheers.

EDIT:

Just had a thought. When the voltage sag was happening, the power was showing as about 500 watts and the voltage something like 32V. The sums add up better with those figures 500W /32v = 15.625 Amps. Since the controller is set to Max 16 A, that makes sense.

Yeah - it'll do for a while yet. Unlike Guerney, I can live with the lower PAS levels. I run 5 PAS levels. Level 1 is set at 20%, Level 2 at 40%, level 3 at 60%, Level 4 at 80% and 5 at 100%

These translate as fractions of 16 amps as:

1 - 3.2 Amps
2 - 6.4 Amps
3 - 9.6 Amps
4 - 12.8 Amps
5 - 16 Amps

These figures assume the battery can actually deliver that and I am pedalling right to keep it delivering and that the bike isn't backing off because it is at the limit.

Since I have the PAS speed settings set to 100% at all PAS levels, it is limited only by the current settings.
 
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Az.

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Apr 27, 2022
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Why sag does bother you so much? As long as battery gets you where you want to be I see no problem.
 

guerney

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Sep 7, 2021
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I was planning a more cautious approach, to spare my aged battery. People like me should not be allowed more power, we'll only use it all the time.

Level 2 is enough to overcome motor resistance, for those rare occasions when I'm running very low on power.


58118


You could future proof? Set levels 1 to 7 for up to 16A, but set higher levels for different batteries.
 
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guerney

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In that video, Guerney seems to be being pursued by maniacs. Looks like if an accident happened someone might be had up for 'Wanton and Furious Driving'. One of the offences mad cyclists can be put in the dock for.

:)

I would have reported them, if I had clearly recorded the number plate. Disguised as a pedestrian, I've seen that car since and the number plate is deliberately obscured, couldn't read it then either.

It was about an hour after closing time, and as I passed the pub they had parked and loitered outside, all of a sudden the headlights switched on and the unhinged screaming and chashing started lol. Could still be a useful recording, as I might use that mad screaming in a pointless sonic experimentation project.

I'd need an entirely different bike to outrun the car.
 
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saneagle

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Thanks for the info and the warning..... But... Why?
You can enjoy your blissful ignorance while it lasts. Just don't eat the forbidden fruit, which will bring you enlightenment.

Sag is a result of Ohms law, where the voltage drop in any conductor is directly proportional to the current flowing through it V =IR.

In the case of sag, the resistance is the internal resistance of each cell. When you have 4 cells in parallel, the current through each cell is the total current ÷ 4. When you have 7 cells in parallel, each cell only has to deal with 1/7 of the total current, so the difference in sag between the two batteries will be in the ratio 7/4. Your 13Ah battery could be 4P or 5P. When you compare with other batteries, you need to consider the number of parallel strings.
 
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guerney

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Sep 7, 2021
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Maybe you should have got a Bafang instead of the other one. I can also Ghost pedal and it flies up hill. Spookey....
I reckon @saneagle chose the TSDZ2B for his secret concentrated wind juice project, hoping a torque sensored mid drive would use less juice than an equivalent rated cadence sensored hub motor.
 
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Ghost1951

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Jun 2, 2024
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You can enjoy your blissful ignorance while it lasts. Just don't eat the forbidden fruit, which will bring you enlightenment.

Sag is a result of Ohms law, where the voltage drop in any conductor is directly proportional to the current flowing through it V =IR.

In the case of sag, the resistance is the internal resistance of each cell. When you have 4 cells in parallel, the current through each cell is the total current ÷ 4. When you have 7 cells in parallel, each cell only has to deal with 1/7 of the total current, so the difference in sag between the two batteries will be in the ratio 7/4. Your 13Ah battery could be 4P or 5P. When you compare with other batteries, you need to consider the number of parallel strings.
You know, I could almost swear it is a 6p battery, not 4 or 5. That ought to reduce the sag at high power.

EDIT:

Yes. Just removed the battery from its holder. Bottom of case is engraved - Sanyo. 13Ahr 10S6P. May 2021.

They must be 2200 ma types or near. six 2200 ma runs out about 13.2 A hr, possibly 2150mahr which comes out just under 13 Ahr. Sanyo made batteries around those numbers.
 
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guerney

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You know, I could almost swear it is a 6p battery, not 4 or 5. That ought to reduce the sag at high power.
Open it up and upload pics? @Nealh might tell you what continuous amps your battery pack is capable of discharging.
 

Ghost1951

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Jun 2, 2024
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@guerney

I don't think I want to disturb it for a reason like that. Thanks for the idea anyway. It's working and I can ride it 25 miles up steep and lengthy hills before i need to stop pulling 16 amps out of it. If I had continued hammering it, it would soon have hit the 31 volt cut off and shut down. As you said above, even then if you go to low PAS setting you can overcome the motor resistance and pedal home. Maybe I should set my lowest PAS setting at less than 20% of max current - say 10% just for that reason. 1.6 amps in no 1 pas would last a lot longer when nursing it home with an empty battery than 3.2 amps which is where it is now. I might also set the percentage for that level way lower than 100% where it is now. Just for that first level.
 
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